Citibank said that the digits were not identified as Social Security numbers, and were printed at the bottom of the envelope along with other numbers and letters that together resembled a mail routing number.
Citibank spokesman Stephen Silverman said the firm regretted the "processing error," which took place in January.
"Although there is little or no risk to our customers, we decided to be completely transparent to our customers by notifying them of the error," Silverman said.
The bank offered customers who were affected by the snafu the option of enrolling in a free credit-monitoring service for up to 180 days.
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